Ab initio calculation of ionization potential and electron affinity of six common explosive compounds

Abstract

The vertical and adiabatic ionization potential (IP V and IP A) and vertical electron affinity (EA V) for six explosives, hexogen (RDX), octogen (HMX), triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), 2,4,6-trinitrotoluene (TNT), and pen-taerythritol tetranitrate (PETN), have been studied using ab initio computational methods. The IP V was calculated using MP2, CBS-QB3, and Koopmans' theory, while the IP A was calculated with B3LYP, CAM-B3LYP, ωB97XD, B2PLYP, and MP2 using the ∆E method for the ground state B3LYP optimized geometries. IP A s of RDX and TNT were also calculated using CBS-QB3 with relaxed geometries of the ions. Of the methods tested, B3LYP and B2PLYPD provided superior and more consistent results for calculating the IP compared to CBS-QB3 level IP A calculations and experimental data (where available). CBS-QB3 was used as a benchmark for calculating the EA V as experimental data has not been reported. For calculations of the EA V , B3LYP performed the worst while MP2 and B2PLYPD predicted values closest to those made by CBS-QB3. Basis set effects were evaluated using 6-31+G(d,p), 6-311+G(d,p), and 6-311+G(3df,2p) for both IP and EA. 6-31+G(d,p) gave satisfactory results for calculating IP while 6-311+G(3df,2p) had improved results for calculating the EA. The four nitro-containing compounds have exothermic reduction potentials while the peroxides are endothermic. In addition, it was determined that RDX, HMX, TATP, and HMTD had unstable geometries in their reduced forms. The results should be useful in developing detection and screening methods including ionization methods for mass spectroscopy and fluorescence quenching methods of detection.